1. Field of the Invention
This invention relates to a forward osmosis method using recyclable driving solutes which are easy to remove and recycle, have a high rejection by forward osmosis membranes, have low toxicity and are cost efficient.
2. Description of the Related Art
Osmosis occurs when two solutions of differing osmolar concentrations are separated by a membrane permeable to the solvent but not to the solutes. In osmosis, water flows spontaneously from the low concentration source solution to the high concentration driving solution. With the abundance of seawater available and the increasing demand for water suitable for drinking and industrial use, seawater desalination is important. Many methods of desalination have been developed including technologies for distillation, reverse osmosis, freezing, electrodialysis, ion exchange, and forward osmosis.
Forward osmosis may be employed for different purposes. For producing pure water from seawater, Murray (“Desalting Seawater with Ammonia, Part 2: Osmosis” Water and Sewage Works, Volume 15, page 525, 1968) suggests an ammonium carbonate driving solution. The ammonium carbonate would be removed from the product water by stripping. Muller (“Fresh Water for Arizona by Salt Replacement Desalination”, Hydrology and Water Resources in Arizona and the Southwest, Volume 4, 127, 1974) suggests a sucrose driving solution. After extraction of water from seawater, the sugar would be combined into larger molecules, thereby lowering the osmotic pressure of the product solution. The solution could then be dewatered by ultrafiltration. The pure water permeate is the final product and the solute molecules would be subjected to enzymatic hydrolysis to become sucrose for recycling. A process using forward osmosis to provide the hydrostatic driving pressure for reverse osmosis desalination is described in Popper et al (“Desalination by Osmosis-Reverse Osmosis Couple”, Science, Volume 159, 1364-1365, 1968). For power production, Loeb (“Production of Energy from Concentrated Brines by Pressure-Retarded Osmosis, I. Preliminary Technical and Economic Correlations,” Journal of Membrane Science, Volume 1, 49-63, 1976; “Production of Energy from Concentrated Brines by Pressure-Retarded Osmosis, II. Experimental Results and Projected Energy Costs”, Journal of Membrane Science, Volume 1, 249-269, 1976) has proposed a “Pressure-Retarded Osmosis” power plant which uses osmosis to produce a solution under a high hydrostatic pressure. The high pressure of the solution can be recovered as electricity with a hydroturbine-generator.
Forward osmosis may be employed in situations which do not require the separation of pure solvent. One application is the concentration or dewatering of a solution where the fate of the solvent is irrelevant. Concentration of fruit juices has been suggested (Sourirajan, Reverse Osmosis, Academic Press, New York, 1970; Loeb and Bloch, “Countercurrent Flow Osmotic Processes for the Production of Solutions Having a High Osmotic Pressure”, Desalination, Volume 13, 207, 1973). Results on fruit juice concentration have been reported by Popper et al. (“Dialyzer Concentrates Beverages”, Food Engineering, Volume 38 (4), 102-104, 1966) and Mizutani et al., (“Osmotic Concentration by Using Reverse Osmosis Membranes”, Journal of Applied Polymer Science, Volume 20, 2305-230, 1976). Wang (Yuma Desalting Test Facility, Yuma, Arizona, Private Communication, 1975) used reverse osmosis followed by forward osmosis for recovery of proteins from whey. For simultaneous dehydration and hydration requirements, Randal et al. (“Application of Product Return Osmosis to Reduce Energy in Beet-Sugar Processing”, Energy Use Conference: Proceedings of the International Conference, R. A. Fassolare and C. B. Smith, eds., Pergamon Press, New York, Volume 1, 837-844, 1977) have proposed osmosis as an energy-saving method for concentrating sugar beet extract while hydrating the molasses by-product.
Another type of situation occurs in irrigation where water containing salt solutes deleterious to plants may be readily available. The forward osmosis method can be used to transfer water from a brackish source solution to a harmless or useful one containing fertilizer, thereby reclaiming a resource that would otherwise be lost (Moody and Kessler, An Initial Investigation into the Use of Direct Osmosis as a Means for Obtaining Agricultural Water from Brackish water, University of Arizona, Internal Report, 1971; Moody and Kessler, “Application of Direct Osmosis: Possibilities for Reclaiming Wellton-Mohawk Drainage Water”, Hydrology and Water Resources in Arizona and the Southwest, Volume 5, 101, 1975). In an analogous situation, forward osmosis can produce emergency potable water for humans in small ocean vessels such as life boats. In that case, the process transfers water from the sea into a concentrated nutrient solution (Kessler and Moody, “Drinking Water from Sea Water by Forward Osmosis”, Desalination, Volume 18, 297-306, 1976; Murray, “Desalting Seawater with Ammonia, Part 2: Osmosis”, Water and Sewage Works, Volume 115, 525, 1968).
The concept of forward or direct osmosis as a practical commercial process has been recognized since at least the 1930's. See, for example, U.S. Pat. No. 2,116,920. This patent discloses the use of a concentrated sugar and CaCl2 aqueous solution to “pull” water out of fruit juices. The general process has been in continuous commercial use to manufacture fruit juice concentrates since at least that time. The concept of a removable “driving solute” in forward osmosis driven separation is disclosed by Charles Moody (Dissertation. School of Renewable Natural Resources The University of Arizona, 1977). He outlines the use of dissolved SO2 as an osmotic agent that would increase an effluent's molality above that of sea water, thereby creating a forward osmotic bias that would cause fresh water migration through a semi-permeable membrane from a sea water influent. The SO2 would then be removed from the effluent by increasing the effluent temperature to drive it out as a gas.
Cath et al. (Journal of Membrane Science, Volume 281, Issues 1-2, 70-87, 2006) state that the concentrated solution on the permeate side of a membrane used in forward osmosis is the source of the driving force in the process. The reference lists different terms to name the concentrated solution and these include draw solution, osmotic agent, osmotic media, driving solution, osmotic engine, sample solution or just brine.
Cath et al. (U.S. Published Patent Application US2006/0144789 and PCT Application PCT/US2007/071141) disclose methods and systems for purifying liquids using at least one forward osmosis unit for diluting a water source for a downstream desalination unit. Cath et al. use a draw solution having a solute concentration close to that of seawater and discloses as useful draw solutions seawater, concentrated seawater, or other suitable hypertonic solutions.
In U.S. Pat. No. 3,617,547, Albert Halff and Allen Reid use an approach similar to that of U.S. Pat. No. 6,391,205. In both cases, an osmotic agent composed of salts, whose solubility is very temperature dependent is used to increase an effluent's molality above that of sea water, thereby creating a forward osmotic bias that would cause fresh water migration through a semi-permeable membrane from a sea water influent. The osmotic agent is removed by lowering the solution temperature to precipitate the solute out of solution. The precipitate is removed, re-dissolved in water aided by heating and then recycled. The water obtained from the filtered precipitate solution could be further purified, or used as is. These processes are encumbered by the energy inefficient need to chill all of the permeate and recycle streams, as well as the need to reheat the recycle.
Keith Lampi et al. in U.S. Pat. No. 6,849,184 describe an approach of obtaining fresh water from impure or sea water by a combination of forward osmosis and reverse osmosis. Salt and sea water are introduced into a chamber with two semipermeable membranes and then sealed; the introduced solution, therefore, has a molality greater in comparison to that of ordinary sea water. Ordinary sea water is then exposed to a first one of the two semipermeable membranes, which causes the sea water solvent, i.e., fresh water, to cross the first membrane. As the fresh water passes through the first membrane and into the sealed chamber, the internal pressure of the sealed chamber increases. The container is constructed such that the second membrane is in a zone of the sealed chamber still containing sea water largely unmixed with the introduced salt. As the sealed chamber internal pressure increases beyond the osmotic pressure of sea water, fresh water solvent is forced through the second membrane via reverse osmosis. The salt/sea water solution in the sealed chamber would then be discarded. Although possibly practical as a survival device, this invention requires the continuous re-supply of salt or highly concentrated salt solution and is not amenable to a continuous process approach.
Hough (U.S. Pat. No. 3,721,621; issued Mar. 20, 1976) discloses a forward osmosis system wherein a second solution solute is removable by precipitation. The second solution solute solubility is dependent upon pH. The Patent discloses carbonates, oxalates, tartrates, and the like, metals such as calcium, strontium, barium, nickel, cobalt, copper, mercury, silver, iron, and the like. The Patent states that the preferred solutes include iron sulfide and/or calcium sulfite.
Glew (U.S. Pat. No. 3,216,930; issued Nov. 9, 1965) teaches a process for liquid recovery and solution concentration that uses mixtures of water and another gas such as for example, sulfur dioxide; or liquid such as for example aliphatic alcohols with from about 4 to about 6 carbon atoms and alicyclic alcohols having from about 4 to about 6 carbon atoms, as draw solutions for forward osmosis.
While there are various systems and driving solutions for utilizing forward osmosis to purify liquids, there still remains a need in the art for a more effective system for the removal of unwanted substances from liquids. The present invention, different from prior art systems, provides a system and novel driving solutions for use in forward osmosis systems.